Source : Webster's Revised Unabridged Dictionary (1913)

Virulent \Vir"u*lent\, a. [L. virulentus, fr. virus poison: cf.
   F. virulent. See {Virus}.]
   1. Extremely poisonous or venomous; very active in doing
      injury.

            A contagious disorder rendered more virulent by
            uncleanness.                          --Sir W.
                                                  Scott.

   2. Very bitter in enmity; actuated by a desire to injure;
      malignant; as, a virulent invective.

Source : WordNet®

virulent
     adj 1: extremely poisonous or injurious; producing venom; "venomous
            snakes"; "a virulent insect bite" [syn: {deadly}, {venomous}]
     2: infectious; having the ability to cause disease [ant: {avirulent}]
     3: harsh or corrosive in tone; "an acerbic tone piercing
        otherwise flowery prose"; "a barrage of acid comments";
        "her acrid remarks make her many enemies"; "bitter words";
        "blistering criticism"; "caustic jokes about political
        assassination, talk-show hosts and medical ethics"; "a
        sulfurous denunciation" [syn: {acerb}, {acerbic}, {acid},
        {acrid}, {bitter}, {blistering}, {caustic}, {sulfurous}, {sulphurous},
         {venomous}, {vitriolic}]